The River South Esk rises on the western slope of Blackhope Scar in the Moorfoot Hills. From the benchmark on the summit, the windmills on Emly Bank and Browbeat Hill can be seen. On its way down hill, the river flows passed the remains of the 16th century Hirendean Castle and there are wonderful views of Midlothian looking over the Reservoir to the Pentlands, Arthur's Seat and across the Forth to Fife.

Gladhouse Reservoir, opened in 1879, is the first of a series of reservoirs built on the South Esk to increase the water supply to the Edinburgh area. At 186 ha (460 acres) it is the largest area of freshwater in the Lothians. It is a Site of Special Scientific Interest and is a winter roost for many wildfowl, particularly thousands of pink footed geese. Although there is a footpath around the reservoir, access to the shoreline is restricted during the breeding season in the spring.

The river then flows into the very deep Rosebery Reservoir which was opened in 1885. These reservoirs along with nearby Edgelaw are popular with fishermen for catches of pike and brown and rainbow trout. The Rosebery Water Treatment Works were built in the 1940s, extended in 1965 and 1989, and are presently being upgraded again. The woodlands around Edgelaw and Rosebery provide some of the heron breeding sites in Midlothian.

When the older reservoirs in the Lothians were no longer adequate to meet the demand for water in the Edinburgh area, the Megget Reservoir in the Borders was built in the 1980s. It supplies 100 million litres of water to the Lothians daily, the water being distributed to Gladhouse and Glencorse Reservoirs for intermediate storage or going directly to various treatment plants in Edinburgh and Midlothian.

The river now flows through the Rosebery Estate where nearby Rosebery House, plain and largely 19th century, was built on the site of the older mansion house of Clerkington. Of more interest is the castellated Rosebery farm steading buildings with a clock tower which is also a doocot. At one time pigeons were farmed for food and were allowed to feed freely in the fields but later their indiscriminate feeding habits of eating crops were seen as a nuisance and they were fed in the yard like poultry. The Earl of Rosebery encouraged farm improvements in the early 1800s as can be seen further downstream at Carrington Mains.